Alternating-current electromagnet



Dec. '9. 1924. 1,518,612

H. M. STEVENS ALTERNATING CURRENT JLECTROMAGNET Filed 001:. 7. 1921Inventor- 1 Harry M. Stevens,

by .M

His Attorney.

Patented Dec. 9, 1924.

UNITED STATES PATENT QFFECE.

HARRY M. STEVENS, OF SCHENECTLDY, NEW YOIaK. ASSIGNOR TO GENERALELECTRIC COMPANY, A CORPORATION OF NEW YORK.

ALTERNATING-CURBENT ELECTROMAGNET.

Application filed October 7,1921. Serial No. 505,991.

To all whom it may concern:

Be it known that I, HARRY M. STEVENS, a citizen of the United States,residing at Schenectad in the county of Schenectady, State of Lew York,have invented certain new and useful Improvements in Alternating-CurrentElectromagnets, of which the following is a specification.

My invention relates to improvements in elect-romagnets for alternatingcurrent circuits and has fo its object the provision of means wherebythe operating characteristics of the electromagnet are improved.

It is well known that an electromagnet when operated on an alternatingcurrent circult is noisy. The noise may vary from a slight hum to achattering noise, due to the fact that the voltage, in periodicallypassing through zero, allows the armature to recede from the magnet.This noise, however, slight, is objectionable and various methods ofovercoming the same have been proposed. One of the commonest methods ofovercoming this difiiculty isbv means of what is known as a shadingcoil. The function of this shading coil is to produce an out of phaseflux which persists, when the main flux passes through zero, and helpsto sustain the armature. Various forms of shading coils have beendevised but in general they consist of a copper ring enclosing a portionof the ole face so as to induce an out of phase on in the area enclosedby the ring.|

This shading coil arrangement has been more or less successful where thepresence of residual magnetism is not objectionable. In many cases,however, it is essential to eliminate the residual magnetism so thatthere will be no tendency for the armature to stick when the current iscut oil or the energy of the magnet is reduced to the. releasing point.It is not diflicult to prevent chattering or humming of the armature ormoving core if the latter makes a metal to metal contact with thestationary core so as to bring about a cond tion known as freezingbetween the two magnetic parts.- Under this condition, however, there isdecided tendency for the parts to stick toget \er a ter the magnetizingforce is removed. One of the objects of my invention is to provide menus whereby, in addition to overcoming the noise, the tendency of theparts in stic due to residual magnetism is overcome.

It has always been assumed that the shaded or out of phase fluxtraverses the same circuit as the main flux. It, therefore, seemedreasonable to suppose that an air gap in the main magnetic circuit wouldbe effective to prevent residual ma netism in the shaded circuit. I havefoun however, that this is not the case in any of the shaded poleconstructions which have come under my observation. I have found thatthe flux of the shaded pole merely traverses a local circuit and thatthe effect of the residual ma"- netism can only be avoided by providingan gapsat the-points in the local or shaded circuit where the out ofphase flux is passing in opposite directions. The provision of such airgaps, however, makes it necessa to insure a greater flux in the shadedcircuit and to locate this flux more symmetrically with respect to themain flux than would be the case if there were no air gap. In carryingout my invention, therefore, I provide an arrangement which in additionto affording the necessary air gaps in the local magnetic circuit alsoinsures a sufiicient shaded flux symmetrically disposed with relation tothe main flux to hold the armature when the main flux passes throughzero, thereby not only preventing chatterin" but also eliminatingresidual magnetism. ather objects and purposes of my invention willappear in the course of the following specification in which I haveshown my invention embodied in concrete form for purposes ofillustration.

In the accompanying drawingillustrating my invention embodied inconcrete form, Flg. 1 is a sectional view of an ClBCLulIlilfnetembodying my improvements; and F g. 2 is a perspective view of a sha edpole lEu-c therefor. Referring first to Fig.1,th mum'w al .10 representsan olectroumgnet invmlml for operation on alternating current "in i"'--.

.This magnet is of the solenoid type and prvided with a stationary poleface 11, the n ternal magnetic circuit bein through lllij frameconstituted by the mem ers 12. 1-3 and 4, this being the usual construcion of an ectromagnet of this character. The movi armatuae or plunu 'erof the electromagnet prises ,rod 15 of magnetic material hav ing at itsd a somewhat smaller rod 16, of non -magnetic material, which extendsthrough the stationary pole piece 11.

Inn

- shaded pole area.

Between the two rods and 16 is a pole piece 17 which constitutes one ofthe important features of my invention. This 'pole piece, which is shownin detail in Fig. 2,

. comprises a disc of copper or other non magnetic material which is anelectrical conductor, secured to the upper end of the rod 15 so as tocooperate with the stationary pole piece 11. This copper disc in generalperforms the function of the usual copper ring used in the shading coilconstruction, that is, it provides a copper path for currents forinducing out of phase flux in the As a matter of convenience inconstruction the magnetic members 18, 19, 20, 21, 22 and 23 are in theform of pins or studs which are securely inserted in the copper disc.This disc is slotted at 24 so as to break the continuity of the ring asa. whole, thereby preventing heating currents from circulating aroundthe ring as a. whole. The disc is also slotted at each one of the pins18, and 22 so as to break the continuity of the copper circuitsurrounding these pins. These pins 18, 20 and 22, therefore, constituteunshaded area. Pins 19, 21 and 23, however, have solid metal around themand, therefore, have out of phase flux induced in them. These shadedpins are symmetrically disposed in the main flux path as shown toproduce an equalized attractive effect to hold the armature in stableequilibrium when the main flux passes through zero. It u ill be notedthat the pins or studs do not extend up so to he tinsh with the mainsurface of the copppr disc but terminate somewhat short of the surfaceso that when the disc is in contact with the stationary pale piece 11there i be an air gap between the pole ends f the studs and thestationary pole piecc In the constructionsshown, this is accoinplishedby counteisinking a ring 2:") in the disc having sufficient width toinclude the ins. The purpose of this air gap is'to eliminate residualmagnetism. I have found that out of phase flux lll(l\1('t' l in theshaded portions does not pass around through the main magnetic circuitconstituted by the members 12, 13 and 14 but merely forms a localcircuit from the studs into the stationary pole piece back-ol' thestuds- An air gap at any other portion of the magnetic circuit istherefore not effective to prevent residual magnetism in this localmagnetic circuit. 1 have, therefore, provided this air gap in the localcircuit by making the studs short enough so that they do not form amagnetic path between the rod 15 and the sationary pole piece 11.

Since an air gap is provided in the local circuit it is necessary toinsure that the out of base flux will be forced across the gap witsufficient intensity to sustain the armature. The purpose of theunshaded portions constituted by studs 18, 20 and 22 is to bring thisabout. Under normal operation the flux will pass between the studs andthe stationary portion and through the outer mag-.

netic circuit When the main flux approaches zero there will be set up alocal flux from one of the shaded portions 19, 21 or 23, through an airgap at the point of egress of the flux to the stationary pole piece andback through an air gap at the point of ingress of the flux into theadjacent nonshaded studs 18, 20 or 22. The presence of the unshadedstuds, therefore, affords a better path for the local flux than will bethe case if the return path for the flux in the local circuit were notpresent. When the magnet is deenergized, the relative direc tion of fluxin the shaded and unshaded portions may be such that there is' atendency to form a local circuit or circuits in a path or paths of suchlow reluctance that the circuits will persist and keep the movingarmature attracted at least temporarily, to the stationary pole piece.By introducing an air gap at the points of ingress and egress of thelocal circuit or circuits through the members, the reluctance of thismagnetic path becomes sufiicient to prevent the persistence of thecircuit. For purposes of illustration I have shown a structure involvingthree local magnetic circuits, symmetrica ly disposed with relation tothe main flux, but of course more or less of such circuits may beemployed without de arting from the spirit of my invention. M yexperience, however, seems to indicate that the number of local magneticcircuits employed may have a bearing upon the operation. Three suchcircuits seemed to result in better operation than any other number thusfar tried although with further experience it might be foundthatimprovcd operation would result from the use of a different number.of circuits. i

While I have described my invention as embodied in concrete form forpurposes of illustration, it should be understood that I do not limit myinvention thereto since various modifications thereof will suggestthemselves to those skilled in the art without departing from the Spiritof n" invention, the scope of which is set forth in the annexed claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is

1. The method of preventing chattering and avoiding residual magnetismin an alternating current magnet having a mo'iabli-f armature whichconsists in supplying a local out of phase flux symmetrically disposedwith relation to the main flux sufficient to hold the armature when themain flux passes through zero and providing an air gap at the points ofingress and egress of the out of phase flux through the armature.

a the points of ingress and e ress of said comprising a plurality ofmagnetic members included in said circuit and forming a pole face forone of said portions, means for setting up local magnetic circuitsthrough said members and the other portion and arr gaps in saidcircuits.

4. An electromagnet for alternating current circuits havi stationary andmovable portions formingt e main magnetic circuit comprising meansassociated with the armature for causing a symmetrically disposed localout of phase flux through a portion of the armature and the stationarymagnetic pole face of the magnet and an air at ocal circuit through thesaid ptfie face.

5. The combination with an electromagnet for alternating currentcircuits and an armature therefor, of a plurality of,mag netic membersforming the pole piece of the armature, a copper conducting pathsurrounding said members and air gaps in the magnetic circuit of themembers.

(3. The combination with an electromagnet for alternating currentcircuits and an armature therefor, of magnetic members forming a.plurality of local out of phase magnetic circuits from the armaturesthrough the starrmwary portion of the mag nets and return and air gapsin said local circuits.

7. The combination with an electroruag net for alternating currentcircuits and an armature therefor, of a plurality of groups of magneticmembers each cooperating with the stationary portion of the magnet toform a local magnetic circuit and air gaps in said local circuits.

8. The combination with an electromagnet for alternating currentcircuits, an armature therefor, of a pole piece comprising a pluralityof pins of magnetic material each embedded in copper which projectsbeyond the pins to form an air gap.

9. The combination with an electromagnet for alternating currentcircuits, an armature therefor, of apole piece comprising a plurality ofins of magnetic material each surrounded similar pins adjacent theretohaving no surrounding copper.

.10. The combination with an electromagnet for alternating currentcircuits, an armature therefor, of a. pole piece comprising a copperdisc and a plurality of pins of magnetic material embedded therein andhaving less depth than the disc so as to form an air 11. The combinationwith an electromagnet for alternating current circuits, an armaturetherefor, of a pole piece comprising a copper disc and a plurality ofpins of magnetic material inserted therein certain of which aresurrounded by cop er while the remainder are not so surroun ed.

12. The combination with an electromagnet for alternating currentcircuits, an armature threfor, of a pole piece comprising a copper discand a plurality of pins of magnetic material inserted therein and havingless depth than the disc, certain of said pins being surrounded bycopper while the remainder are not so surrounded.

y copper and a plurality of- 13. The combination with an electromag-HARRY M. STEVENS.

